The present invention relates to an engine combustion controller that selects an optimal one of combustion modes, which include lean combustion and stoichiometric combustion. More particularly, the present invention pertains to an engine combustion controller that feedback controls the air fuel ratio of an air fuel mixture when stoichiometric combustion is performed based on the composition of the exhaust gas.
In a conventional engine, the air fuel ratio of an air fuel mixture is feedback controlled so that the air fuel ratio becomes stoichiometric. A stoichiometric air fuel ratio enables a catalytic converter to effectively purify exhaust gas. When controlling the air fuel ratio, the fuel concentration of the mixture is adjusted based on an output signal of an air fuel ratio sensor arranged in a passage of the exhaust gas.
When the air fuel ratio is feedback controlled, an abnormality in the fuel supply system may cause an undesirable air fuel ratio. This would hinder appropriate feedback control of the air fuel ratio. In such case, the components of the exhaust gas are not in the desirable state. Thus, the catalytic converter cannot purify the exhaust gas efficiently. This may result in undesirable emissions.
To prevent such undesirable emissions, Japanese Unexamined Patent Publication No. 5-26085 describes a system for detecting abnormalities in a fuel supply system. The system determines the occurrence of an abnormality in the fuel supply system when a feedback compensating value of the air fuel ratio obtained during the air fuel ratio feedback control is outside a normal value range. Then, the system warns the driver of the abnormality.
In recent years, direct cylinder injection type engines have become popular. A combustion controller for such engine selects different combustion modes to improve fuel efficiency. The combustion mode is selected from stoichiometric combustion, in which the air fuel mixture is stoichiometric, and lean combustion, in which the air fuel mixture is more lean than the stoichiometric state. Lean combustion is performed when the operating state of the engine is stable.
For accurate detection of abnormalities, the engine must be in a stable operating state for a certain time period. Lean combustion is commenced when the engine enters such stable operating state. The air fuel ratio is feedback controlled during stoichiometric combustion, and abnormalities of the fuel supply system are detected during the feedback control. Thus, it is difficult to detect abnormalities in the fuel supply system when the engine is in a stable operating state.
Accordingly, lean combustion may be prohibited when checking for abnormalities in the fuel supply system. However, this would lower fuel efficiency.
It is an object of the present invention to provide an apparatus for controlling combustion in an engine that accurately and easily detects an abnormality in the fuel supply system.
To achieve the above object, the present invention provides a combustion controller of an engine having a fuel supply system for supplying fuel. The combustion controller selects a combustion mode performed by the engine from either one of lean combustion, in which an air fuel mixture leaner than a stoichiometric mixture is burned, and stoichiometric combustion, in which a stoichiometric mixture is burned, in accordance with an operating state of the engine, and feedback controls the air fuel ratio of the air fuel mixture when the stoichiometric combustion is performed using an air fuel ratio feedback compensating value, which is set in accordance with a component concentration in an exhaust gas. The controller includes a preliminary testing means for testing the fuel supply system and determining that the fuel supply system is in a normal state when one of a plurality of predetermined first conditions is satisfied. The first conditions include a condition using the variable feedback compensating value. A lean combustion prohibiting means prohibits the lean combustion when the preliminary testing means determines that the state of the fuel supply system is ambiguous. A main testing means tests the fuel supply system and determines that the fuel supply system is in an abnormal state when one of a plurality of predetermined second conditions is satisfied. The second conditions include a condition using the variable feedback compensating value.
A further aspect of the present invention provides a method for controlling combustion in an engine, which has a fuel supply system for supplying fuel, by selecting a combustion mode performed by the engine from either one of lean combustion, in which an air fuel mixture leaner than a stoichiometric air fuel mixture is burned, and stoichiometric combustion, in which a stoichiometric air fuel mixture is burned, in accordance with an operating state of the engine, and feedback controlling an air fuel ratio of the air fuel mixture when the stoichiometric combustion is performed. The method includes setting an air fuel ratio feedback compensating value in accordance with a component concentration in an exhaust gas when executing feedback control during stoichiometric combustion, and preliminarily testing the fuel supply system and determining that the fuel supply system is in a normal state when one of a plurality of predetermined first conditions is satisfied. The first conditions include a condition using the feedback compensating value. The method further includes prohibiting the lean combustion when the state of the fuel supply system is determined to be ambiguous in the preliminary testing step, and main testing the fuel supply system and determining that the fuel supply system is in an abnormal state when one of a plurality of predetermined second conditions is satisfied. The second conditions include a condition using the feedback compensating value.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.